Hi, I've been using Adobe Acrobat to create PDF's from scanned images (converted to 4 bits with Photoshop), but I've recently discovered that when I use the feature "Optimize Scanned PDF", images are converted to 8 bits, and the resulting PDF has a bigger size.

So I need a software that features a filter like that from Acrobat (deskew, despeckle, etc to facilitate compression) but supporting 4 bit images, or an alternative software, or a stand alone filter.